This study investigated the effect of a repeated and standardized jaw protrusion training (JPT) task on corticomotor excitability as assessed by motor evoked potentials (MEPs) in masseter and tongue muscle with the use of transcranial magnetic stimulation (TMS). Sixteen healthy participants performed three series of a standardized JPT task on three consecutive days. Each day participants performed 41-min of JPT consisting of three series. In all series, participants were instructed to target 50% and 100% of the maximum jaw protrusion positions. In the first and third series without any feedback but during the second series, participants were provided a custom-made mandibular advancement device to help achieve the correct protruded position. Single pulse TMS was applied to elicit MEPs from right masseter, right tongue and right first dorsal interosseous muscles (FDI) (as control), pre and post-task on Day-1 and -3. Masseter MEPs and tongue MEPs were significantly dependent on stimulus intensity (P<0.001) and on task session (P<0.001). Amplitude of masseter and tongue MEPs at post-task Day-3 were significantly higher compared to baseline values (pre-task Day-1) (P<0.005). FDI MEPs were dependent on stimulus intensity only (P<0.001) but not on task session (P=0.677). Our novel findings suggest that participants performing an active and repeated JPT task demonstrate neuroplasticity in terms of increased corticomotor excitability not only in masseter muscles but also in tongue muscles. This finding may have implications for patients with obstructive sleep apnea treated by a mandibular advancement device where the lower jaw is passively held in a protruded position. (248 words).
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